1. Field of the Invention
The invention relates generally to the field of magnetic data storage devices, and particularly to read/write heads for use in such devices. The invention particularly provides a new magneto-resistive thin film head for use in disk data storage devices for use in digital data processing systems.
2. Description of the Prior Art
A typical modern digital data processing system comprises a hierarchy of memory devices, including a semiconductor main memory of relatively small capacity, and one or more mass storage devices, which have a much greater capacity than the main memory, but which are also relatively much slower. The mass storage devices provide a back-up store for data which is in the main memory, and also for the voluminous amounts of data which will not fit into the main memory, but which can be called upon by the processor when it is needed. A processor typically only obtains information directly from the main memory, and so, when it needs information which at the moment is only in a mass storage device, it enables the mass storage device to copy the information into the main memory. Some time later, after it has processed the information, the processor enables the processed information to be stored in the mass storage device. This frees up storage in the main memory so that other information may be stored there.
Typical mass storage devices store information on spinning magnetic disks, the information being recorded in the form of transitions in magnetic flux on the magnetic surface of the disk. In particular, the data is recorded in a plurality of tracks, with each track being a selected radial distance from the center of the disk. A read/write head flies in close proximity to the disk surface and is held in the appropriate radial position over the disk by an arm. Under the control of the system's processor unit the arm can move the read/write head to the appropriate track in which the data is recorded so that it may be read, or into which the data is to be written.
A read/write head comprises two pole pieces formed from a magnetic material and a wire coil. At one end, the pole pieces are touching and at the other end there is a slight gap between the pole pieces The head is positioned so that the gap is directed towards the disk surface. When electric current is impressed on the coil, a magnetic flux is generated, which is impressed upon the pole pieces. At the gap, the magnetic flux is directed through the magnetic material in the adjacent disk surface to thereby impress magnetic flux therein.
When data is being written onto a disk, the coil is energized with a varying voltage pattern which corresponds to the data to be written. The varying voltage results in the generation of a corresponding pattern in the magnetic flux which the head applies to the surface of the rotating disk. Since the disk moves relative to the head, the magnetic flux on the disk surface also varies along the length of the arc traversed by the head on the disk.
When the data is read, the head flies over the arc of the disk surface in which the data was written. A small amount of flux from the disk permeates the head. The flux in the head varies in response to the pattern of flux recorded on the disk. The varying flux results in the generation of a varying voltage in the coil, which, in turn, is sensed as the previously-recorded data.
One problem with a conventional read/write head is that the variation in the voltage induced in the coil does not directly follow the actual flux, but instead follows the rate of change of the flux as the disk rotates adjacent the head. It is therefore evident that reading of data with conventional heads is sensitive to the speed of the disk relative to the head, that is, the speed of rotation of the disk.
Recently read/write heads have been developed which include a strip of magneto-resistive material, such as a nickel iron alloy. One such alloy is commercially known as "Permalloy". The strip is positioned in the gap between the pole pieces that are adjacent the disk. In such heads, the electrical resistance of the magneto-resistive material is related to an applied magnetic field. As flux from the disk permeates the head while it flies over the disk surface, the flux is applied to the magneto-resistive material. Thus, the resistance of the magneto-resistive material varies in response to the variations in the flux in the head, which in turn reflects the variations in the flux on the disk. The resistance of the magneto-resistive strip is sensed by conventional sensing circuits to provide a signal that is related to the recorded flux. Thus, unlike the conventional read/write heads, the voltage signals from such read/write heads, specifically from the magneto-resistive strip, are not sensitive to the speed of the disk.
In a head having a magneto-resistive strip, the strip is formed so as to have a magnetization along the length of the strip; that is, the magnetic dipoles in the strip are aligned parallel to the strip's longitudinal axis. A current is applied longitudinally to the strip. A graph of the resistance of the strip to electric current, in relation to the direction of the strip's magnetic dipoles, is a bell-shaped curve. For example, if the strip is a nickel iron alloy, if no external flux is applied to the head, the resistance exhibited by the strip to current applied in a longitudinal direction through the strip (which is parallel to the magnetization) will be at a maximum. If, however, external magnetic flux is applied to the strip which forces the strip's magnetic dipoles into an orientation perpendicular to the length of the strip, the strip's resistance to the applied current will be at a minimum.
Otherwise stated, continuing with the same example, if the current flow is parallel to the magnetization of the strip, the resistance of the strip is at a maximum, but if the current flow is orthogonal to the magnetization, the resistance is at a minimum. Intermediate these two extremes, that is, with the strip's magnetic dipoles aligned approximately forty-five degrees with the direction of the applied current, the change in resistance of the strip with respect to the applied magnetic field is approximately linear. It will be appreciated that the alignment of the strip's magnetic dipoles is related to the applied magnetic flux, and thus the resistance of the strip will be related to the direction and amount of applied magnetic flux.
There are two problems with heads using magneto-resistive strips as read elements. One problem is that the magneto-resistive strip requires external biasing to force it into the linear region so that the resistance changes as an approximately linear function of the applied flux. If a magneto-resistive strip is not biased, a small applied flux from a disk will be unable to change the orientation of the strip's magnetic dipoles sufficiently to provide a large enough change in the resistance of the strip. The same will occur if the strip is biased too much, so that the magnetic dipoles are perpendicular to the strip's longitudinal direction. In either case, the strip will have a very low sensitivity to the applied flux level.
U.S. Pat. No. 4,535,375, issued to G. Mowry, et al., on Aug. 13, 1985, entitled Magneto-resistive Head, discloses a head with a complex magneto-resistive read element. The magneto-resistive element disclosed in that patent includes an elongated magneto-resistive strip and plurality of equipotential strips disposed along the element at a skewed angle (generally, approximately forty-five degrees) with respect to the elements longitudinal axis. A bias current is applied and the equipotential strips force the current to flow generally orthogonal to the strips. This bias current generates a bias field which orients the magneto-resistive strip's magnetic dipoles at a forty-five degree angle with respect to the current.
Another problem with magneto-resistive elements is a result of the tendency of an element, which was originally magnetized in a single magnetic domain (that is, a region in which all of the magnetic dipoles are oriented in a common direction), to develop a plurality of separate magnetic domains. One cause of formation of multiple domains is end effects, that is the perturbation of the dipoles at the ends of the strip, which are usually not precisely aligned with the longitudinal axis because of spreading typical at the end of a magnetic member. Over time, the effect may spread throughout the strip, resulting in multiple magnetic domains throughout the strip.
Another source of multiple domains in magneto-resistive strips arises from the fact that, during writing, the head, including the magneto-resistive strip, is saturated with magnetic flux (hereafter "write flux"). This write flux is perpendicular to the longitudinal direction of the strip, and the magnetic dipoles in the strip tend to align with the applied write flux. After the write operation is completed, the strip's magnetic dipoles return to an orientation along the strip's longitudinal axis, but they need not return to their former orientations. They may instead be aligned in the opposite direction. As this repeatedly occurs following write operations, a number of domains develop with differing orientations of magnetic dipoles. Thus, multiple domains may be created throughout the strips, not just at the ends.
A result of the development of the multiple magnetic domains is Barkhausen noise, which is noise in the voltage signal will be noisy due to sudden jumps in the magnetization of the strip The magneto-resistive element disclosed in the aforementioned U.S. Pat. No. 4,535,375 proposes to minimize Barkhausen noise by providing a very long magneto-resistive strip and sensing the change in resistivity across only a small portion of its length. This can help with minimizing the Barkhausen noise due to end effects, but it does not significantly reduce the noise due to the creation of multiple domains along the strip caused by the write flux applied to the strip. Furthermore, the length of the magneto-resistive element disclosed in the patent and the placement of the element adjacent the disk in the pole tip combine to effectively limit the inter-track spacing, as the tracks must be far enough apart so that, while the element Is positioned over one track, it does not receive interfering flux from an adjacent track.